The present invention relates to a fiber rendering method and MRI (magnetic resonance imaging) apparatus, and more particularly to a method and MRI apparatus for properly rendering brain white matter fibers obtained by diffusion tensor imaging.
FIG. 21 is a flow chart showing a conventional fiber rendering method.
At Step P1, an MR image in an axial or oblique plane is produced from three-dimensional image data collected by a diffusion tensor method or another imaging method (T1- or T2-enhanced or the like) in an MRI apparatus, and the MR image is displayed.
At Step P2, an operator specifies a two-dimensional region of interest R1 (or a three-dimensional volumetric region of interest) on a displayed MR image G1, as shown in FIG. 22.
At Step P3′, regular grid points are generated in the region of interest R1 (or in the volumetric region of interest) as shown in FIG. 23, and they are defined as tracking start points S1, S2, S3, . . . .
At Step P5, one of the tracking start points is selected.
At Step P6′, diffusion tensor analysis is performed on the selected tracking start point in the three-dimensional image data collected by the diffusion tensor method in the MRI apparatus to determine the direction of the principal axis vector, i.e., the direction of the first eigenvector.
At Step P7, if a point at a unit distance along the direction of the principal axis vector falls within the three-dimensional image data space, the point is defined as a neighbor point and the flow proceeds to Step P8′; and if the point falls outside the three-dimensional image data space, the flow proceeds to Step P11.
At Step P8′, data at the neighbor point is created by interpolation or the like on the three-dimensional image data, and diffusion tensor analysis is performed to determine the direction of the principal axis vector and the FA (fractional anisotropy) value.
At Step P9, if the FA value is equal to or more than a threshold, the flow goes back to Step P7 to continue the fiber tracking because the fiber tracking has not reached an end portion of a brain white matter fiber; and if the FA value is less than the threshold, the flow proceeds to Step P11 to terminate the fiber tracking because an end portion of a brain white matter fiber has been reached.
In this way, Steps P7-P9 are repeated until no more three-dimensional image data are found or the fiber tracking has reached an end portion of a brain white matter fiber, and a fiber is tracked from the tracking start point S1 to a neighbor point N1, N2, N3, . . . , as exemplarily shown in FIG. 24. At that time, connectivity is decided by using a scalar product of vectors, for example.
At Step P11, points from the tracking start point to the last neighbor point are saved as one brain white matter fiber.
At Step P12, if any tracking start point not selected at Step P5 remains, the flow goes back to Step P5; otherwise, proceeds to Step P14′.
At Step P14′, an image of the saved brain white matter fibers as viewed in a desired view direction is produced and displayed, as exemplarily shown in FIG. 25.
A diffusion tensor and a nerve fiber extending direction are described in, for example, “Microstructural and Physiological Features of Tissues Elucidated by Ouantitative-Diffusion-Tensor MRI” by Peter J. Basser and Carlo Pierpaoli, Journal of Magnetic Resonance, Series B 111, pp. 209-219 (1996), and in “Diffusion Anisotropy—2D and 3D images of Brain White Matter Fibers—” by Yasuomi Kinosada (Kyoto Prefectural University of Medicine, Department of Radiology), the 30 th Meeting of MR Imaging Study Group, Sep. 4, 1998, at Sapporo, Japan.